The present invention relates generally to uninterruptable power supplies. Specifically it relates to a hybrid rotary-static uninterruptable power supply.
Machinery, computers, and the like used in all areas of business and personal use require reliable power in order to operate effectively under all conditions. When machinery or computers are used to perform tasks that are critical to ongoing work, the public utility power source may not be sufficient. If the utility power source fails, important operations may not be performed, or important or sensitive computer data may be lost. To cope with such power source failure, uninterruptable power systems (also known as UPS systems) are used to provide power. These systems vary widely in structure and operation.
One type of known UPS system is a rotary UPS system having an AC motor generator set and an auxiliary DC motor mechanically coupled to the shaft of the motor generator set. This auxiliary DC motor may be used as a DC generator to charge the backup battery bank during the normal utility powered operation. When a power outage is detected, DC power from the battery bank is connected to the auxiliary DC motor to turn the shaft of the AC motor generator set. Such UPS systems require frequent maintenance. For example, the commutator on the auxiliary DC motor has a limited lifetime. Carbon dust from the brushes of the auxiliary motor can cause shorting problems for sensitive electronic equipment located in the area of the UPS system. Further, when the UPS system switches from utility powered operation to battery powered operation, speed regulation is poor due to the slow response time of the auxiliary DC motor. If the load is changed, the slow response results in significant frequency shifts. The auxiliary DC motor is heavy, and a firm base and extra space is required for coupling it to the motor generator set. Accordingly, the rotary UPS system will be dimensionally longer and heavier than other motor generator sets.
A second type of UPS system is a hybrid UPS system. Three variations of hybrid UPS systems are available.
An off-line hybrid UPS system powers the AC motor of a standard motor generator set from the utility power during normal operation. When a utility power failure is detected, an inverter powers the AC motor of the motor generator set. The inverter is normally inactive, and is switched on at or near the moment of utility failure. During normal utility powered operation, off-line systems have low stress on their semiconductor components. However, when the utility fails, maximum semiconductor stress is incurred. This moment of utility failure is the most critical instant for a UPS system, and if the inverter fails, it is most likely to fail at this time. The probability of failure is increased since the conditions immediately prior to a utility power outage are often unusual and unpredictable and may include power surges, asymmetrical transients, or reverse power flow. Because of line conditions at switchover after utility failure, a large transient often occurs. Such a transient can overload or disable the inverter. Since the inverter is normally off-line and inactive until absolutely needed, there is no way to be certain in advance whether the inverter will be able to be successfully activated and ready to assume the load upon utility failure. Further, from the time of detection of utility failure to the response of switching on the inverter, output frequency to the load is continuously declining.
A second hybrid UPS system is known as an on-line system. An on-line system has a utility powered inverter assembly which is active continuously even during normal utility powered operation. The inverter continuously drives the AC motor of a standard motor generator set. At the time of utility failure, no additional stress is placed on the on-line system, since the inverter's power may be drawn from a battery instead of from the utility, and the inverter is already running. Problems with this on-line hybrid UPS system are that inverter losses are incurred in addition to losses from the motor generator set, degrading system efficiency. Also, the rectifier portion of the inverter assembly, which charges the batteries, is continuously exposed to line transients, and contributes to line current distortion. The inverter must handle all line conditions when it runs continuously in this fashion. Line transients and line current distortion can cause overloads of the inverter as well as affect the quality of the power supplied. Also, when the inverter is run continuously from the utility, the inverter generates undesirable harmonics back into the line.
A third type of hybrid UPS system is a parallel processing system. In this system, an inverter is idled in parallel with the utility. When a utility failure occurs, the utility is disconnected and the AC motor of the motor generator set is run from the inverter. This system has efficiency comparable to a solitary motor generator set. However, this system must use pulse width modulation in order to avoid excessive current harmonics. As in an online system, the inverter is continuously exposed to line transients. In order to protect the inverter, especially during faults and failures, additional protection and filtering are required.
It would be desirable to provide a UPS system which has the advantages of being isolated from line transients, having a quick response time between utility failure and switchover, and having frequency maintenance upon utility failure, efficiency of operation, and reliability at the critical time of utility failure. It would also be desirable to provide such a system without adding excessive costs or space requirements.